THE PHYSICS OF THE FAR-INFRARED-RADIO CORRELATION. I. CALORIMETRY, CONSPIRACY, AND IMPLICATIONS

Abstract

(Abridged) The far-infrared (FIR) and radio luminosities of star-forming\ngalaxies are linearly correlated over a very wide range in star formation rate,\nfrom normal spirals like the Milky Way to the most intense starbursts. Using\none-zone models of cosmic ray (CR) injection, cooling, and escape in\nstar-forming galaxies, we attempt to reproduce the observed FIR-radio\ncorrelation over its entire span. We show that ~2% of the kinetic energy from\nsupernova explosions must go into primary CR electrons and that ~10 - 20% must\ngo into primary CR protons. Secondary electrons and positrons are likely\ncomparable to or dominate primary electrons in dense starburst galaxies. We\ndiscuss the implications of our models for the magnetic field strengths of\nstarbursts, the detectability of starbursts by Fermi, and cosmic ray feedback.\nOverall, our models indicate that both CR protons and electrons escape from low\nsurface density galaxies, but lose most of their energy before escaping dense\nstarbursts. The FIR-radio correlation is caused by a combination of the\nefficient cooling of CR electrons (calorimetry) in starbursts and a conspiracy\nof several factors. For lower surface density galaxies, the decreasing radio\nemission caused by CR escape is balanced by the decreasing FIR emission caused\nby the low effective UV dust opacity. In starbursts, bremsstrahlung,\nionization, and Inverse Compton cooling decrease the radio emission, but they\nare countered by secondary electrons/positrons and the decreasing critical\nsynchrotron frequency, which both increase the radio emission. Our conclusions\nhold for a broad range of variations on our fiducial model.\n